To perform many ophthalmic surgical procedures, it is essential that the optical properties of the cornea of the eye be known. It is especially important that those optical properties defining the visual defect known as astigmatism (i.e., the power and angle of orientation) be known to properly perform cataract surgery, intraocular lens implantation, radial keratomoty and other ophthalmic surgeries. Accurate information regarding the power and angle of orientation of the astigmatism is essential if the surgeon is to suture the cornea during the surgical procedure in such a manner as to minimize any astigmatism.
Keratometers and ophthalmometers are widely used for measuring the radius of curvature of a cornea. Known keratometers typically reflect a light beam off the convex mirror-like surface of a moist cornea and then process or manipulate the reflected corneal image so as to provide information regarding the astigmatic condition of the eye. Examples of such known keratometers are disclosed in U.S. Pat. Nos. 4046463, 4165744 and 4355871.
Since ophthalmic surgery involves very precise, delicate, and minute incising and manipulation of the corneal tissue, such surgery is generally performed while viewing the cornea through an ophthalmic surgical microscope. In response to this approach to ophthalmic surgery, other keratometers have been developed for use with surgical microscopes. In these keratometers, a reflected corneal image passes through the keratometer before reaching the objective lens of the microscope. See, for instance, U.S. Pat. Nos. 4157859, 4429960 and 4439025.
These and other prior art keratometers typically suffer from at least one of several problems. First, it is frequently difficult (when using the prior art keratometers) to measure the radius of curvature of the cornea with as great a degree of accuracy as may be desired. Second, in the known devices a secondary light source (separate from the light source of the microscope) is generally required to provide the light beam that is reflected off the cornea to generate the reflected corneal image. This secondary light source adds to the cost of the keratometer and often intrudes into the operating work area. Third, the prior art devices are frequently slow or cumbersome to operate and are generally highly complex and, as a result, fairly expensive. Fourth, conventional keratometers frequently provide no means for conveniently determining, in diopters, the degree of astigmatism of the cornea. Finally, known keratometers typically intrude into the surgical operating zone, thereby complicating the surgical procedure.
A prototype comparative surgical keratometer was designed and tested by Herbert F. Butler III as his undergraduate thesis at Massachusetts Institute of Technology ("MIT"). This keratometer is described in the paper "Comparative Surgical Keratometer Report and Zeiss Implementation Proposal" presented at MIT by Mr. Butler on Oct. 25, 1982. The Butler keratometer includes an illuminator assembly for creating an annular light beam to be reflected off the cornea, a comparator assembly for creating a plurality of annular light beam "standards", and means for superimposing a selected annular light beam "standard" on the reflected corneal image. In surgical application, the reflected corneal image and the superimposed "standard" image are transmitted through the microscope for viewing by the surgeon. Using the relative relationship of the superimposed images as a guide, the surgeon sutures the cornea so as to ensure that the reflected corneal image (initially elliptical to the extent that any astigmatism is present) directly overlays and matches the annular "standard" image, thereby eliminating any astigmatism which may have been present.
It was proposed by Butler that the annular light beam transmitted for reflection off the cornea be created by folding a laser beam off an aluminized annulus forming part of the illuminator assembly. The annulus would be formed and positioned to ensure the reflected image intersects the cornea. In implementation, Butler used a circular flourescent light in place of the aluminized annulus.
The Butler keratometer tends to suffer from several problems. First, no means are provided for quantitively measuring the angle of inclination of the astigmatism of the cornea. Second, the illuminator and comparator assemblies are each believed to have separate light sources and to each use a series of optical lenses. By providing separate light sources and lenses for the illuminator and comparator assemblies, the complexity and hence cost of the device is increased. Third, conventional surgical microscopes (with which the Butler keratometer is intended to be used) apparently require substantial modification in order to accept the Butler device.